Frequency divider network



March 21, 1944.

FM Waxed M. G. CROSBY FREQUENCY DIVIDER NETWORK Filed March 29, 1941INVENTOR arm 5 9 BY 7 /w ATTORNEY meme Mar. 21,

scram umrco sTA'rEs PATENT 7 or-Pics f FREQUENCY DIVIDER NETWORK MurrayG. Crosby, Riverhead, N. 1., assignor to Radio Corporation of DelawareofAmcrica,

a corporation Application Mar h 29, 1941, Serial No, 385,800

Claims. .(cl. ass-2o) sence of the high frequency voltage whose ire- Vis normally so 'unstable'that it may be easily held in step at asub-harmonic of the frequency'to be divided. This type of frequencydivider network has the disadvantage for some purposes thatosciliations. are produced even in the absence of the controlling inputwaves.

Accordingly, it may be stated that it is one of the important objects ofthe present invention to provide a frequency division network which isprovided with means to prevent output oscillations at the dividedfrequency in the absence of the alternating input voltage whosefrequency is to be divided.

' Another important object of the invention may will not be developed atthe aforesaid converter output circuit unless the center frequency ofthe applied modulated carrier waves is very close in frequency to theconverter input circuit frequency and unless the amplitude of theconverter input voltage is above a predetermined amplitude level whichcorrespondsto a level above an undesired be stated to reside in theprovision of a frequency dividing circuit system of a superheterodyneconverter network and a frequency multiplier network, the multiplierfunctioning to provide for the converter network oscillations whosefrequency differ from the converter input wave frequency by thefrequency of the converter output circuit.

Frequency division networks are desirable for use in receivers ofangular velocity modulated carrier waves, such as phase, or frequency,modulated carrier waves, and such frequency'division networks have beendisclosed in my U; S. Patent No. 2,064,106, granted December 15, 1936,as well as in my U. S. Patent No. 2,230,231, granted February 4, 1941.In the circuits of these patents, prior to demodulation of the modulatedcarrier wavesthe latter are subjected to the frequency division in sucha manner that the band width of the modulated carrier wave is reduced inthe same proportion as the center frequency of the modulated carrierwave, As pointed out in the said patents, such frequency division isparticularly advantageous in the reception of phase, or frequency,modulated carrier waves because the signal to noise ratio at the inputof the demodulator is greatly improved the narrower the band width ofthe demodulation network input circuit. It may be stated, therefore,that it is another object of my invention to provide a frequencydivision network prior to the demodulator of a receiver of angularvelocity modulated waves, the division network comprising a converterwhose output circuit is resonant to a predetermined fractional frequencyof the center frequency of i the converter input circuit, and afrequency multiplier noise level.

Still other objects of the invention are to improve generally theefficiency and reliability of frequency division networks in receiversof fre quency modulated carrier waves, and more particularly to providedependable frequency division networks having limiting action, and whichare economically manufactured and assembled in frequency modulationreceivers.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically a circuit organization whereby my invention may becarried into effect.

Referring now to the drawing there is shown the frequency divisionnetwork of my present invention, and it will be understood that it maybe considered, for a specific utilization, as being located between theintermediate frequency amplifler network and the demodulation network ofa frequency modulation receiver of the superheterodyne type. Forexample, let it be assumed that the superheterodyne receiver is oneemployed in the presently assigned frequency modulation band of 42 to 50megacycles (mc.), and that the first detector reduces the centerfrequency of the modulated carrier waves to an intermediate frequencychosen from a range of 2 to 5 me. In

such case many advantages are secured if a frequency division network isutilized to reduce the center frequency, say 4 mc., by a desired factorwhich is an integer. As explained in my aforementioned patents, the bandwidth is simultaneously decreased by the same factor. In other words, ifthe center frequency is reduced by a factor of 4, then the band widthwould be reduced by the same factor of 4. Since the maximum permissiblecenter frequency deviation in use at present in broadcast FM receptionis kilocycles (kc.) to either side of the center frequency, it will beseen that reducing the center frequency by a factor of 4 will result ina reduction of the band width from 200 kc. to 50 kc. By such frequencydivision and simultaneous band width reduction the input circuit of thefrequency modulation detector may be adjusted for the reduced band widthwith a consequent improvement in signal to noise ratio at the detectorinput circuit.

Specifically considering the circuit shown on the drawing, let it beassumed that numeral l designates the usual intermediate frequencytransformer coupled to the plate circuit of the last intermediatefrequency amplifier tube. In that case each of the primary and secondaryresonant circuits 2 and 3 would be tuned to the center frequency F. Asstated before, and merely by way of specific illustration, F is assumedto have a value of 4 me. As is well known the center or carrierfrequency is deviated at the transmitter in dependence upon theamplitude of the modulation signal, and the rate of frequency deviationis dependent upon the modulating frequencies themselves. In the assumedcase the permissible band width of transformer 1 would be 200 kc.

The numeral 4- designates the converter tube which may be of the6SA'7type, although the invention is not restricted to this type of tube norto a tube of the pentagrid type. Generally, the tube may comprise acathode 5 and an output electrode, or plate, 6. Between the cathode 5and plate 6 are arranged in sequential relation a Signal input grid 1, apositive screen grid 8, and a second signal, or oscillation, inputelectrode 9, a positive screen grid to and the usual suppressor grid II.The cathode 5 is connected to ground through the customary biasingresistor l2 shunted by an intermediate frequency by-pass condenser. Thelow potential side of secondary circuit 3 is established at groundpotential, while the opposite side is connected to the signal grid 1.The electrode 9 is connected through resistor B to the grounded end ofbiasing resistor l2.

The plate 6 is connected to the positive terminal of a direct currentsource through a resonant output circuit comprising a coil 14 shunted bycondenser IS. The circuit I l-l5 is tuned to a It will be understoodthat N may be any factor which is an integer. The signal modulatedcarrier voltage developed across circuit l5--|4 may be transmitted toany utilizing means through the condenser l6 which has a low impedanceto the carrier voltage. As explained before, it is desired to divide thecenter frequency of the modulated carrier waves and securesimultaneously proportional reduction of the band width or frequencydeviation. Hence, the effective band width of the energy in circuitl4--l5 would be reduced N times. If the effective band pass width attransformer l was 200 kc., and N equalled 4, then the pass band width attuned circuit l5-l4 would be 50 kc. The utilizing means can be any wellknown type of frequency modulation detector which functions to derivethe modulating signal from the modulated carrier wave. Reference is hadto my aforesaid patents for disclosure of networks which may be usedfollowing circuit l5l 4.

The frequency multiplier tube is designated by numeral 20, and it may bea tube of the pentode type such as one of the 68K? type. The cathode 2|of this tube includes a self-biasing resistor 22 connected to ground andshunted ,by an appropriate by-pass condenser. The input grid 23 isconnected by the coupling condenser 24 to the high potential side ofcircuit l 4-l 5, the grid being connected to ground through the leakresistor 25. The plate 30 of tube is connected by lead 3| and couplingcondenser 32 to the grid 9. The plate 30 is also connected to the highpotential side of a resonant circuit comprising a coi .0

and a shunt condenser M, the low potential side of this circuit beingconnected to a positive terminal of an appropriate direct currentsource.

Circuit 41l4l is tuned to a frequency which is equal to In the specificillustration given, the resonant f requency of circuit it-4| would be 5me. The biasing resistor 22 is given a value such that, in the absenceof input voltage to grid 23, direct current voltage developed acrossresistor 22 is suflicient preferably to bias grid 23 so as to adjust thegrid close to plate current cut-off. It will now. be realized that theresonant frequency voltage developed across the circuit 40- is impressedupon the electrode 9 simultaneously with the impression of the frequencymodulated carried wave energy of center frequency F upon grid 1.

The-frequency modulated carrier wave voltages impressed on respectivegrids 1 and '9 are heterodyned by virtue of electron coupling, and

produce the beat frequency voltage whose center frequency is in circuit15-!4. Hence, it will be seen that the output of the converter tube istuned to the desired subharmonic frequency, and that this converteroutput voltage is fed to the input electrode of a frequency multipliertube. In the latter circuit the sub-harmonic frequency receives amultiplication of Nil times, and this multiplied output is then fed tothe electrode 9 of the converter tube 4 so that the output of theconverter consists of the heterodyne beat between the center frequency Fand the frequency of the voltage developed across circuit 40-. It willbe seen under these circumstances an oscillation will exist in circuitl5l4 when the converter gain and the multiplier gain exceed unity. Theconverter gain depends upon the strength of the signal applied from theinput transformer I so that in the absence of signal energy at the inputtransformer I there will be no output voltage at circurt l5-l4 sincethere will be no oscillations produced through the multiplier tube.Tubes 4 and 20 cooperate with the associated tuned circuits to provide are-entrant circuit 1. e., a regenerative modulation circuit. Theconstants of the tubes 4 and 20 are chosen so that when the signal inputlevel exceeds a predetermined amplitude then the re-entrant actionbecomes sufficient oscillation.

Additionally, the network of this invention may have its constantsadjusted so as to provide the characteristics of a limiter stage. Withsuch adjustment, when the signal input voltage is raised above the levelrequired to start oscillations the output voltage does not appreciablyincrease. Accordingly, when used to divide the frequency, or phase,deviation in a frequency, or phase, modulation receiver, the divisionnetwork also performs the function of a limiter network. Therefore, theusual limiter stage employed prior to the detector may be dispensedwith.

A further advantage of this type of frequen y divider, when employed ina frequency, or phas modulation receiver, is in the selective c ithereof. Due to the resonant frequencies of the to sustain circuitsemployed in the regeneration circuit, there is a limited range offrequency P which will cause the system to oscillate and'produce anoutput voltage. Thus, the receiver may be tuned to the proper centerfrequency F so as to produce oscillations in the divider, andinterfering signals which do not'have the frequency F, but whichrejected.

A still further advantage which this division circuit has for use infrequency, or phase, modulation receivers is its threshold action whichcan be utilized to eliminate noise from the receiver in the absence ofsignals. The usual frequency modulation receiver produces a roar ofnoise when there is no signal present. This is due to the fact that thelimiter increases the gain of the receiver so as to amplify'the noise tofull volume. However, with the present type of frequency divider-limiternetwork, the gain of the receiver may be adjusted so that the thresholdof the divider is just above the noise level. Hence, there will be nooutput voltage unless a signal is present which is stronger than, andover-rides, the noise level. It will, therefore, be appreciated that thepresent frequency division network improves the signal to noise ratio atthe demodulator input circuit by virtue of a conjoint action of thedivision of the band width of the frequency modulated carrier waves andthe threshold action of the network itself. Furthermore, by virtue ofthe improved selectivity and the limiting action distortion is greatlyminimized.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of my invention, as set forth in the appended claims,

What! claim is:

1. In a frequency division network, a tube provided with at least anelectron emission element, an output electrode and at least two coldelectrodes located in the electron stream to said output electrode, asource of frequency modulated oscillations of a predetermined meanfrequency and predetermined frequency deviation connected to one of saidcold electrodes, an output circuit connected to said output electrode,said output circuit being tuned to a subharmonic frequency and means forapplying to said second cold electrode of said first tube frequencymodulated oscillations developed across said second output circuit, thegain at said "source being chosen sufficiently high so that said firsttube threshold is come through the usual selective circuits, will .be

above the noise level whereby oscillations of mean frequency are notdeveloped in said first tube output circuit in the absence of frequencymodulated oscillations at said source whose amplitude overrides saidnoise level, and said first tube additionally functioning as anamplitude limiter-in response to said oscillations of mean frequencybeing produced.

2. In a frequency modulated carrier wave reception system, an inputcircuit tuned to the center frequency F of said modulated carrier waves,said input circuit having a pass band sufliciently wide to pass arelatively wide range of frequency deviation of the modulated waves, anelectron discharge tube provided with at least two signal inputelectrodes, said input circuit being connected to one of said signalinput electrodes-a modulated carrier voltage output circuit for saidtube which is tuned to a subharmonic of said center frequency, means formultiplying modulated carrier oscillations produced across said outputcircuit so as to produce further modulated carrier oscillations whosecenter frequency is equal to F I (Nd: U

and means for applying said last named oscillations to the second signalinput electrode of said tube thereby to produce in said output circuitand whose frequency deviation is reduced from said wide deviation by N;the gain of the system at said input circuit being chosen to provide athreshold for said tube above the noise level there by to preventproduction of oscillations in said 1output circuit for input waves belowsaid noise eve 3. In a frequency division network for a frequencymodulated carrier wave receiver of the superheterodyne type, a tubeprovided with at least an electron emission element, an output electrodeand at least two cold electrodes located in the electron stream to saidoutput electrode, an intermediate frequency-tuned input circuitconnected to one of said cold electrodes, an output circuit connected tosaid output electrode, said output circuit being tuned to a sub-harmonicfrequency i ,where F equals said intermediate frequency and N is aninteger, said output circuit having a pass band width which is N timesnarrower than the pass band width of the said input circuit, a secondtube provided with at least an electron emission element, an outputelectrode and an input electrode, said second tube b g connected as anon-oscillating multiplier, means for impressing upon said second tubeinput electrode voltage of said sub-harmonic frequency, a second outputcircuit connected trode, said second a frequency and means for applyingto said second'cold electrode of said first tube voltage developedacross said second output circuit and the gain of the receiver at saidtuned input circuit being so chosen that the threshold of the first tubeis just above the noise level, the output circuit voltage remainingsubstantially constant in response to the intermediate frequency energyincreasing above said threshold level thereby to provide an amplitudelimiting function.

4. In a frequency modulated carrier wave ireception system, an inputcircuit tuned to the center frequency F of said modulated carrier waves,an electron discharge tube provided with at least two signal inputelectrodes, said input circuit being connected to one of said signalinput electrodes, a modulated carrier voltage output circuit for saidtube which is tuned to a subharmonic of said center frequency and whosepass band width is reduced N times relative to the input circuit passband width, the gain of the system at said input circuit beingsufliciently high to permit said tube to have a threshold above thenoise level, means for multiplying modulated carrier oscillationsproduced across said output circuit so as to produce frequency modulatedcarrier voltage whose center frequency is equal to to said second tubeoutput elecoutput circuit being tuned to and means for applying saidlast named voltage 0 limitingcharacteristic whereby said oscillationsproduced in said output circuit remain substantially constant inamplitude in response to said oscillations of means frequency beingproduced.

5. In a frequency modulated carrier wave reception system, an inputcircuit tuned to the center frequency F of said modulated carrier wavesandhaving a wide pass band, an electron discharge tube provided with atleast two signal input electrodes, said input circuit being connected toone of said signal input electrodes, a modulated carrier voltage outputcircuit for said tube which is tuned to a subharmonic.

of said center frequency and having a pass band reduced N times in widthfrom the input pass band, means for multiplying modulated carrieroscillations produced across said output circuit so as to producefrequency modulated carrier voltage whose center frequency is equal to aF (Nd: 1)

means for applying said last named voltage to the second signal inputelectrode of said tube, and the gain of said system at said inputcircuit being so chosen that no modulating carrier voltage is developedacross said output circuit until a modulated carrier wave amplitude atsaid input circuit exceeds a predetermined noise level and said tubehaving an amplitude limiting characteristic such that voltage at saidoutput circuit remains substantially constant after the wave amplitudeat the input circuit exceeds the noise level.

MURRAY G. CROSBY.

